Production of hydrogen peroxide

ABSTRACT

The invention relates to hydrogen peroxide manufacture, and catalyst therefor, by direct oxidation of hydrogen with oxygen in an acidic aqueous medium. The catalyst includes a Group VIII metal on a partially hydrophobic, partially hydrophilic support, such as Pd on fluorinated carbon. Improvements in H2O2 selectivity and catalyst stability are achieved by adding a source of sodium and chloride ions to the reaction medium and, in the case of a fluorinated carbon support, adding a source of fluoride ions.

FIELD OF THE INVENTION

This invention relates to a process for the production of hydrogenperoxide by direct catalytic oxidation of hydrogen with oxygen. Theinvention also relates to a catalyst for such process and a method forproducing the catalyst.

BACKGROUND OF THE INVENTION

Hydrogen peroxide is commercially produced using a process known as theRiedl-Pfleiderer process. In accordance with this two step process,anthraquinone in a carrier solvent, termed "working solution", is cycledbetween an oxidation reactor and a hydrogenation reactor to converthydrogen plus oxygen to hydrogen peroxide. Variations to the processhave concentrated on the form of anthraquinone, the composition of theworking solution and the type of catalyst used. A typical catalyst ispalladium, raney nickel, or nickel boride on an inert support. Thecatalyst may be in the form of a slurry or a fixed bed. Hydrogen isneeded at high partial pressures in this reaction posing the risk ofexplosion. The process is characterized as being complex and capitalintensive.

Processes for the direct oxidation of hydrogen and oxygen to hydrogenperoxide offer the opportunity to develop a simpler, less costlyprocess. Processes of this nature have been proposed, but to theinventors' knowledge have not been commercialized to date. Thedifficulties with the heretofore proposed processes include:

low concentrations of product

low selectivity (thus high hydrogen consumption)

low reaction rates

hazardous operating conditions (particularly hydrogen partial pressurerequirements in the explosive range) and

high acid content.

Exemplary of such processes are the following patents, all of whichinclude catalytic conversion of hydrogen with oxygen in an acidicaqueous medium:

U.S. Pat. No. 4,009,252 issued to Izu et al. reports good productconcentrations (9-12% H₂ O₂ by wt.) by operating at high acidconcentrations (1 gpl HCl plus 49 gpl H₂ SO₄) using Pd deposited onsilicic acid, and oxygen to hydrogen molar ratios of 1.5 to 20, wellinto the explosive range for hydrogen. Selectivities for hydrogen tohydrogen peroxide were good with many examples in the range of from80-89%. Reaction rates were generally low, ranging from less than 1 tojust over 6 g of hydrogen peroxide per liter-hour.

U.S. Pat. No. 4,661,337 issued to Brill reports high concentrations ofhydrogen peroxide and high reaction rates using Pd deposited on carbonin an aqueous solution containing 35 gpl HCl, by operating a stirredreactor in such a manner to keep the thickness of the aqueous slurry to2 mm or less. For example, concentrations of 19.5% hydrogen peroxidewere achieved at a rate of 48 g of hydrogen peroxide per liter-hourusing hydrogen at 250 psi partial pressure and oxygen at 750 psi totalpressure. However, much of the benefit of the higher reaction rates waslost since most of the reaction vessel was empty. Also, the reactionconditions were in the explosive range for hydrogen.

U.S. Pat. No. 4,772,458 issued to Gosser et al. (see also U.S. Pat. No.4,681,751 and EPA 0132294 to Gosser et al.) achieved high concentrationsand reaction rates with moderate selectivity at low acid levels (lessthan 2.5 gpl H₂ SO₄) using Group VIII metals on a variety of carriers,but at hydrogen concentrations of 17% or higher, making the processhazardous. Selectivities tended to be low, ranging from 30% to 70%,provided bromide ions were present in the reaction medium. If chlorideions were used, very low selectivities of about 6% were achieved. Thebest results appear to have been achieved using a 1:10 ratio of Pt to Pdon an alumina carrier (1.10% total metal) with a hydrogen concentrationof 17.8%. Hydrogen peroxide concentrations were 16.4% at 70% selectivityand the reaction rate was 52 g hydrogen peroxide per liter-hour.

There is a need for a direct oxidative process for the production ofhydrogen peroxide which will produce hydrogen peroxide in goodconcentrations and at high selectivities and reaction rates, whileallowing the process to be conducted at low acid levels and below theexplosive range of hydrogen.

SUMMARY OF THE INVENTION

The present invention is based on a number of surprising discoveriesmade in investigating the direct catalytic oxidation of hydrogen withoxygen in an acidic aqueous medium using a catalyst comprising a GroupVIII metal on a support. Firstly, the inventors discovered that thenature of the catalytic support used was important. Typically supportsused in prior art processes were either strongly hydrophobic or stronglyhydrophilic. The inventors discovered that a hydrophilic/hydrophobicbalance in the catalyst support (and thus the resulting catalyst) wasdesirable. The catalyst (and catalyst support) must be partiallyhydrophobic so as to allow the gaseous reactants (hydrogen and oxygen)to contact the catalyst surface. However, the catalyst (and catalystsupport) must also be partially hydrophilic, or partially wettable, soas to allow the hydrogen peroxide formed at the catalyst surface to bediffused into the liquid phase. If the hydrogen peroxide remainsassociated with the catalyst surface for a period of time water isformed.

The inventors have found that this hydrophobic/hydrophilic balance ispreferably achieved using a fluorinated carbon support or a partiallywettable Vulcan carbon support. The level of fluorination is preferablyin the range of 10-65% F, more preferably 20-50% F.

A second surprising discovery was that the selectivity of the reactionfor hydrogen peroxide could be increased with the addition of a sourceof sodium and chloride ion. This can be achieved in the catalystpreparation stage, as will be described hereinafter, or by adding asource of these ions to the acidic aqueous reaction medium. In factsince these soluble ions are constantly removed with the aqueousreaction medium during the process, a supply of these ions to theaqueous medium is preferable throughout the process or at least once adecline in catalytic activity is noticed. The most economical source ofthese ions is in the form of NaCl. Amounts in range of 3 to 30 wt %based on catalyst are desired.

The inventors noticed that the catalytic activity of their preferredcatalyst (Pd on a fluorinated carbon support) declined with use. Havingdiscovered the level of fluorination to be important to the catalyst,the inventors tried adding a source of fluoride ions to the aqueousmedium. This led to an important third discovery, that a source offluoride ions in the aqueous medium stabilized the catalyst againstdecline in catalytic activity. A convenient source of fluoride ions isNaF, which can be included in amounts of 2 to 10 wt % based on catalyst.

In producing a supported catalyst, the inventors made a fourth importantdiscovery. The inventors found that it was preferable to slurry togetherthe Group VIII metal (preferably Pd) with sodium citrate in a solutionsuch as water. It is believed that this forms a Pd-sodium citratecomplex or colloid with two important consequences. When the catalystsupport is impregnated with the Pd-sodium citrate complex, the metal isstrongly held to the support and well distributed on the supportsurface. In the preferred embodiment of the invention, this method ofcatalyst preparation also provides the desired sodium and chloride ionsin the catalyst, the sodium being supplied from the sodium citrate andthe chloride from the chloride salt of the Group VIII metal (for examplePdCl₂) which is initially slurried with the sodium citrate.

The combination of the above-described discoveries have resulted in aprocess for the production of hydrogen peroxide which can be conductedwith good concentrations of H₂ O₂ (5-6%), at high selectivities (up to100%) and good reaction rate (5-11 gpl-hr H₂ O₂), while allowing one tooperate at hydrogen pressures below the explosive limit and moderateacidities (for example 6 gpl H₂ SO₄).

Broadly stated, the invention provides a process for producing hydrogenperoxide by direct oxidation of hydrogen with oxygen in an acidicaqueous medium, comprising:

(a) contacting the hydrogen and oxygen containing acidic aqueous mediumwith a catalyst consisting of at least one Group VIII metal on apartially hydrophobic, partially hydrophilic support in a pressurevessel;

(b) providing a source of sodium and chloride ions to the acidic aqueousmedium either at the outset of the reaction or once there is a declinein catalytic activity;

(c) maintaining the pressure in the vessel in the range of 3.5 MPa-20MPa, with a hydrogen partial pressure below the explosive limit; and

(d) maintaining the temperature in the range of the freezing point ofthe aqueous medium to about60° C.

In another aspect, the invention broadly provides a catalyst for use inthe production of hydrogen peroxide, comprising:

a) a partially hydrophobic, partially hydrophilic support, preferablyvulcan carbon or fluorinated carbon with a 10-65% F content;

(b) a Group VIII metal; and

(c) a source of sodium and chloride ions.

In yet another broad aspect of the invention, there is provided a methodof producing a catalyst for the production of hydrogen peroxide,comprising:

(a) providing sodium citrate and a Group VIII metal salt in an aqueoussolution;

(b) heating the solution to form a Group VIII--sodium citrate colloid;

(c) adding a catalyst support to the colloid containing solution;

(d) evaporating the solution from the solid; and

(e) reducing the resulting solid in a hydrogen atmosphere.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A Group VIII metal is used in a catalytically effective amount in thecatalyst of this invention. While such metals as Pt, Ru, Rh, Ir arecatalytically active for the production of hydrogen peroxide, Pd is thepreferred metal. Mixtures of Group VIII metals may also be used. Themetal is generally provided in the form of salt, preferably a chloridesalt such as PdCl₂.

The Group VIII metal is employed in the form of a supported catalyst,the catalyst support being partially hydrophobic and partiallyhydrophillic, as described hereinafter.

The support should have a surface area in the range of 50 m² /g to 1500m² /g. A surface area of about 130 m² /g has been found to be suitable.Preferably, the support is used as discrete particles or granules(particle size less than 1 micrometer being suitable), but it may alsobe deposited on other support material such as ceramic beads or rings,as is known in the art.

As previously set forth, the catalyst support (and the resultingcatalyst) should have a hydrophobic/hydrophilic balance which allows thegaseous reactants (H₂ +O₂) to reach the catalyst surface (in aqueousmedium) while allowing the formed H₂ O₂ to be released into the aqueousmedium. Strongly hydrophobic catalyst supports as are known in the art,are not suitable. Hydrophobicity is often defined by the "contact angle"according to Young's Theory. A catalyst support having a contact angleof 90° is typically accepted as being a hydrophobic catalyst support.Catalyst supports in accordance with the present inventions will have acontact angle less than 90°.

Two preferred catalyst supports in accordance with this invention arepartially wettable prefluorinated carbon and Vulcan carbon. In respectof the former material, the level of fluorination affects thehydrophobic/hydrophilic nature of the catalysts. A level of fluorinationof 10-65% F is preferred. A level of fluorination of 20-50% F is morepreferred with 28% F being found to be sufficient. Partially wettableVulcan carbon is a specially treated activated carbon available fromCabot, U.S.A.

The catalyst is preferably made by first preparing a complex or colloidof the Group VIII metal with sodium citrate. This provides a strongerattachment of the metal to the catalyst support and better disperses themetal on the catalyst surface. To that end, sodium citrate and the GroupVIII metal are slurried in a solution such as water and heated to formthe colloid. Heating should be at the boiling point for at least 6 hoursand preferably 10 hours. The amount of Group VIII metal used should besufficient to provide about 0.1-10% wt in the final catalyst. In respectof Pd, an amount of 0.7% wt in the catalyst is sufficient.

The catalyst support is impregnated with the metal-colloid solution.Preferably a reagent is added to the catalyst support metal-colloidslurry to lower the density of the slurry and decrease the tendency ofthe catalyst support to float at the surface. Methanol is suitable forthis purpose. After slurrying, the solution is evaporated and thecatalyst is reduced in a hydrogen atmosphere (preferably 14 hours at300° C.).

In accordance with the preferred embodiment described above, thecatalyst inherently contains the desired sodium chloride ions found toimprove subsequent H₂ O₂ production. The sodium is provided by thesodium citrate while the chloride is provided from the PdCl₂ salt. Whenprepared in this manner, the catalyst can initially be used withoutadding NaCl to the reaction medium.

Production of Hydrogen Peroxide

The process for producing hydrogen peroxide is preferably performed in astirred, pressure reactor such as a flow slurry autoclave, attemperatures between the freezing point of the liquid medium and about60° C., preferably 0°-25° C. As the reaction is highly exothermic,cooling to these temperature is generally needed.

The reactor is preferably charged with the catalyst and the additives(NaCl and NaP, if desirable) prior to adding the acidic aqueoussolution. As previously indicated, these additives may be added laterduring the reaction, once the catalyst activity begins to decline. Theadditive NaCl is preferably added in an amount of 3-30 wt % (based oncatalyst) and the NaF additive is preferably added in an amount of 2-5wt % (based on catalyst).

The acidic solution is preferably a mild acidic solution. An H₂ SO₄solution is economical. An acid strength of 0.5-1.0 % w/w H₂ SO₄ issuitable. Higher acid strengths have not been found to improve theprocess.

Oxygen and hydrogen gas are then charged to the reactor. A majoradvantage of the process of this invention is that it can be carried outat a hydrogen partial pressure below the explosive limit. This limit isunderstood to be the highest percent hydrogen in the reaction atmospherewhich will indicate an explosive range as measured by a standard MSAexplosimeter. Typically a H₂ partial pressure below about 4 volumepercent is used. The total pressure in the reactor will be in the rangeof 500 psig (3.5 MPa) to 3000 psig (20 MPa), the preferred range being1000 psig (6.7 MPa) to 1500 psig (10 MPa). Oxygen may be supplied in apure form or, more preferably, in combination with nitrogen. Oxygencontents as low as air may be used. A preferred gas feed to the reactorconsists of 3.2% H₂, 10% N₂ and 86.8% O₂.

The reaction may be performed on a continuous or batch basis. Since theNaCl and NaP additives are water soluble, these additives should beadded on a continuous basis as they are washed out of the system.

This invention is further illustrated in the following examples:

PREPARATION OF CATALYST Example 1

Sodium citrate (8.07 g) was dissolved in 807 ml of water, to which wasadded 56 ml of 6.7×10³¹ 3 M PdCl₂. This mixture was further diluted with403 ml of water. The mixture was heated to boiling for 10 hours to forma Pd-sodium citrate colloid solution. To this was added 2 g fluorinatedcarbon (fluorine content 28%, median particle size less than onemicrometer, surface area 130 m² /g) together with 100 ml methanol. Thesolution was evaporated and the solid was reduced in hydrogen for 14hours at 300° C. The resultant catalyst contained approximately 0.7% Pd.The catalyst was a partially wettable, black, slightly sticky powder.

Example 2

Further catalysts were prepared in accordance with the procedure set outin Example 1, with fluorinated carbon supports similar in all otherrespects, but having 10% and 65% F content respectively.

Example 3

A further catalyst was prepared in accordance with the procedure set outin Example 1, but using a partially wettable vulcan Carbon supportavailable from Cabot, U.S.A. (Vulcan 9 A32 CS-329).

PRODUCTION OF HYDROGEN PEROXIDE Example 4

A stirred, 450 ml flow slurry autoclave was charged as follows:

0.3 g catalyst (Example 1)

0.03 g NaCl

50 ml 0.6% w/w H₂ SO₄

The autoclave was put in a cold bath maintained at 0° C. The hydrogenand oxygen gas were introduced into the autoclave and the pressure wasincreased to 1000 psig with a total gas flow rate of 300 ml/min (3.2%vol H₂, 10% N₂ and 86.8% O₂), with vigorous mixing. Product conversionand selectivity after 1, 3, 6 and 10 hours were analyzed. The gas phasewas analyzed by on-line gas chromatography with a thermal conductivitydetector. Argon was used as a carrier gas for analysis. The H₂, N₂ andO₂ in the gas feed were separated by a 10'×1/8" diameter stainless steelcolumn packed with 80-100 mesh Porapak QS.

The liquid product was titrated by potassium permanganate toquantitatively determine the H₂ O₂ formed. The equation for thetitration is:

    5H.sub.2 O.sub.2 +2KMnO.sub.4 +3H.sub.2 SO.sub.4 →2MnSO.sub.4 +K.sub.2 SO.sub.4 +8H.sub.2 O+5O.sub.2

The H₂ O₂ concentration was measured directly by titration and confirmedby U.V. spectroscopy. The H₂ conversion was calculated as a ratio of##EQU1## The H₂ O₂ selectivity was calculated on the basis that, if allthe H₂ reacted was converted to H₂ O₂, the selectivity would be 100%,thus ##EQU2##

The results are summarized in Table 1

                  TABLE 1                                                         ______________________________________                                        Reaction H.sub.2 O.sub.2 conc, H.sub.2 O.sub.2                                Time     % w/w       H.sub.2 conv, %                                                                         Selectivity %                                  ______________________________________                                         1 hr    1.1         70        84                                              3 hr    2.3         61        73                                              6 hr    3.8         58        63                                             10 hr    5.4         52        59                                             ______________________________________                                    

Example 5

This example is included to show the results of H₂ O₂ production withoutthe NaCl additive. The catalyst obtained after several runs inaccordance with Example 4 was thoroughly washed and filtered to removeNaCl. When the washed catalyst was thereafter used in H₂ O₂ production(same conditions as Example 4, no added NaCl) the results after 10 hourswere 1.32% w/w H₂ O₂, H₂ conversion 25.5%, H₂ O₂ selectivity 30%.

Example 6

The stabilizing effect of NaP is illustrated in this example. Theprocedure for producing H₂ O₂ set forth in Example 4 was repeated.Without the addition of NaP, after 8 days reaction, the H₂ conversionhad dropped to 33%. When NaF was added to the aqueous medium in anamount of 0.01 g. the H₂ conversion after 8 days was 44%.

Example 7

The importance of the hydrophobic/hydrophillic balance in the catalystsupport is ,illustrated in this example. The catalysts of Example 2 (10%and 65% F content) were subjected to reaction conditions similar toExample 4 with the following results after 10 hours.

                  TABLE 2                                                         ______________________________________                                                H.sub.2 O.sub.2 conv,  H.sub.2 O.sub.2                                % F     % w/w        H.sub.2 conv, %                                                                         Selectivity %                                  ______________________________________                                        10% F   2.1          25        66                                             65% F   2.2          31        38                                             ______________________________________                                    

Example 8

This example illustrates the effect of varying the amount of NaCl addedto the reaction medium. The catalyst of Example 1 (0.7% w/w Pd onfluorinated carbon support) was reacted under conditions similar toExample 4 (0.3 g catalyst, 50 ml 1% w/w H₂ SO₄, varying amounts of NaCl,3.2% H₂, 10.0% N₂ balanced by O₂, 0° C., 1000 psig, 300 ml/min gas, 10 hreaction time). The results are summarized in Table 3.

                  TABLE 3                                                         ______________________________________                                                 H.sub.2 O.sub.2 conc, H.sub.2 O.sub.2                                NaCl(g)  % w/w       H.sub.2 conv, %                                                                         Selectivity, %                                 ______________________________________                                        0.0117   5.83        61        53                                             0.0306   5.83        53        60                                             0.0500   5.86        53        61                                             0.1008   5.79        48        69                                             ______________________________________                                    

Example 9

This example illustrates the effect of varying the amount of NaP addedto the reaction medium. The procedure of Example 4 was repeated, butwith 0.0261 g NaCl and 0.0054 g NaP. After 6 hours, 4.0% w/w H₂ O₂obtained H₂ conversion was 61% and H₂ O₂ selectivity was 63%. Thisprocedure was repeated with 0.0328 g NaCl and 0.0078 g NaF. After 6hours, 3.32% w/w H₂ O₂ was obtained, H₂ conversion was 58% and H₂ O₂selectivity was 60%. This procedure was repeated with 0.03 g NaCl and0.0290 g NaF. After 10 hours, the H₂ O₂ concentration was 2.16% w/w, H₂conversion was 52% and H₂ O₂ selectivity was 23.6%.

Example 10

This example demonstrates that NaBr and KBr do not provide similarbenefits to the NaCl or NaF additives of this invention. The procedureof Example 4 was repeated using 0.0361 g KBr in place of NaCl (acidsolution was 1% w/w H₂ SO₄). After 10 hours, 1.1% w/w H₂ O₂ wasobtained, H₂ conversion was about 4% and H₂ O₂ selectivity was estimatedat 100%. This procedure was repeated with 0.0308 g NaBr in place ofNaCl. After 10 hours reaction, 1.1% w/w H₂ O₂ was obtained, H₂conversion was about 3% (below the detection limit of GC) and H₂ O₂selectivity was estimated at about 100%.

Example 11

The example illustrates H₂ O₂ production with an alternate catalystsupport, partially wettable Vulcan carbon. The catalyst of Example 3 wasreacted under the conditions of Example 4 with the results of Table 4.

                  TABLE 4                                                         ______________________________________                                        Reaction H.sub.2 O.sub.2 conc, H.sub.2 O.sub.2                                Time     % w/w       H.sub.2 conv, %                                                                         Selectivity, %                                 ______________________________________                                         1 hr    1.6         91        99                                              3 hr    4.3         61        100                                             6 hr    5.8         55        95                                             10 hr    6.5         55        64                                             ______________________________________                                    

We claim:
 1. A process for producing hydrogen peroxide by directoxidation of hydrogen with oxygen in an acidic aqueous medium,comprising:(a) contacting said hydrogen and oxygen containing acidicaqueous medium with a catalyst consisting of at least one Group VIIImetal on a partially hydrophobic, partially hydrophilic support in apressure vessel; (b) providing a source of sodium and chloride ions tothe acidic aqueous medium either at the outset of the reaction or oncethere is a decline in catalytic activity; (c) maintaining the pressurein the vessel in the range of 3.5 MPa-20 MPa, with a hydrogen partialpressure below the explosive limit; and (d) maintaining the temperaturein the range of the freezing point of the aqueous medium to about 60°C.;wherein the catalyst support is fluorinated carbon with a level offluorination such that it is partially hydrophobic to allow the gaseousreactants to contact the catalyst, while being partially hydrophilic todiffuse the formed hydrogen peroxide from the catalyst to the aqueousmedium; and wherein the level of fluorination is in the range of about10 to 65% F.
 2. The process as set forth in claim 1, wherein the levelof fluorination is in the range of about 20 to 50% F.
 3. The process asset forth in claim 1, wherein the level of fluorination is about 28% F.4. The process as set forth in claim 1, wherein the Group VIII metal isPd.
 5. The process as set forth in claim 2, wherein the Group VIII metalis Pd.
 6. The process as set forth in claim 1, wherein sodium, chlorideand fluoride ions are provided to the aqueous medium in the form of NaCland NaF.
 7. The process as set forth in claim 5, wherein sodium,chloride and fluoride ions are provided to the aqueous medium in theform of NaCl and NaF.
 8. The process as set forth in claim 7, whereinNaCl is provided in the range of about 3-30 wt %, based on catalyst andNaF is provided in the range of about 2-10 wt %, based on catalyst. 9.The process as set forth in claim 1, wherein the catalyst support ispartially wettable Vulcan carbon.
 10. The process as set forth in claim1, wherein the aqueous medium is agitated to prevent the catalyst fromfloating at the surface.